by the term unity of type; or by saying that the several parts and
organs in the different species of the class are homologous. The whole
subject is included under the general name of Morphology. This is the
most interesting department of natural history, and may be said to be
its very soul. What can be more curious than that the hand of a man,
formed for grasping, that of a mole for digging, the leg of the horse,
the paddle of the porpoise, and the wing of the bat, should all be
constructed on the same pattern, and should include the same bones, in
the same relative positions? Geoffroy St. Hilaire has insisted strongly
on the high importance of relative connexion in homologous organs: the
parts may change to almost any extent in form and size, and yet they
always remain connected together in the same order. We never find, for
instance, the bones of the arm and forearm, or of the thigh and leg,
transposed. Hence the same names can be given to the homologous bones
in widely different animals. We see the same great law in the
construction of the mouths of insects: what can be more different than
the immensely long spiral proboscis of a sphinx-moth, the curious
folded one of a bee or bug, and the great jaws of a beetle? yet all
these organs, serving for such different
purposes, are formed by infinitely numerous modifications of an upper
lip, mandibles, and two pairs of maxill . Analogous laws govern the
construction of the mouths and limbs of crustaceans. So it is with the
flowers of plants.
Nothing can be more hopeless than to attempt to explain this similarity
of pattern in members of the same class, by utility or by the doctrine
of final causes. The hopelessness of the attempt has been expressly
admitted by Owen in his most interesting work on the Nature of Limbs.
On the ordinary view of the independent creation of each being, we can
only say that so it is; that it has so pleased the Creator to construct
each animal and plant.
The explanation is manifest on the theory of the natural selection of
successive slight modifications, each modification being profitable in
some way to the modified form, but often affecting by correlation of
growth other parts of the organisation. In changes of this nature,
there will be little or no tendency to modify the original pattern, or
to transpose parts. The bones of a limb might be shortened and widened
to any extent, and become gradually enveloped in thick membrane, so as
to serve as a fin; or a webbed foot might have all its bones, or
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